1. Field of the Invention
The present invention relates to a steering angle detector for detecting the steering angle or steering direction of a steering wheel mounted on an automobile and, particularly, to a steering angle detector having the function of detecting the absolute angle from the neutral position of a steering wheel.
2. Description of the Prior Art
A rotary encoder is generally used as a steering angle detector for a steering wheel. The rotor member of the rotary encoder is turned in cooperation with a steering wheel to detect the steering angle or steering direction of the steering wheel based on two different pulse signals having a phase difference of 90xc2x0 output from the rotary encoder. Since the steering wheel is turned clockwise or counterclockwise from the neutral position a plurality of times, the steering angle detector must have the function of detecting the absolute angle from the neutral position of the steering wheel.
A rotary encoder which is turned in cooperation with a steering wheel and has a ratchet gear provided with a rotary switch for detecting the absolute angle from the neutral position of the steering wheel has been known as the steering angle detector having the above function. This conventionally known steering angle detector is constituted such that a pair of projections are formed at the periphery of the rotor member of the rotary encoder with a predetermined space therebetween, a plurality of ratchet teeth are formed alternately at two different levels of the ratchet gear, and only one of the ratchet teeth at a higher level projects into the turning locus of the projections. Therefore, according to the steering angle detector thus constituted, when the rotor member makes one revolution in cooperation with the steering wheel, the ratchet tooth contacts the projection of the rotor member to turn the ratchet gear by one tooth intermittently, thereby making it possible to detect the absolute angle from the neutral position of the steering wheel based on an output signal from the rotary switch which changes according to the rotation angle of the ratchet gear.
However, in the above-described steering angle detector of the prior art, every time the rotor member of the rotary encoder makes one revolution, the projection contacts the ratchet tooth to turn the ratchet gear by one tooth intermittently, thus making an impact sound offensive to the ear every time the projection contacts the ratchet tooth. Since the ratchet gear turns by one tooth intermittently to switch the contact point of the rotary switch every time the rotor member makes one revolution, although it can be detected how many revolutions the steering wheel has made based on the output signal of the rotary switch, the absolute angle within one revolution of the steering wheel cannot be detected. Thus, the resolution of absolute angle detection is low.
It is an object of the present invention which has been made in view of the above situation of the prior art to provide a steering angle detector which can reduce noise and improve the resolution of the absolute angle detection of a steering wheel.
To attain the above object, the steering angle detector of the present invention includes a rotation member which turns in cooperation with a steering wheel, first detection means for outputting a pulse signal corresponding to a rotation angle of the steering wheel in cooperation with a cord board mounted to the rotation member, a screw shaft having a spiral groove in an outer surface, a movable member which is engaged with the spiral groove to move linearly in an axial direction of the screw shaft, and second detection means for detecting an absolute angle from a neutral position of the steering wheel in cooperation with the movable member, wherein a first gear formed on the rotation member always meshes with a second gear formed on the screw shaft.
In the steering angle detector thus constituted, when the rotation member turns in cooperation with the steering wheel, the first detection means detects information on the rotation of the steering wheel in cooperation with the cord plate mounted to the rotation member, the movable member moves linearly in the axial direction of the screw shaft which turns in constant mesh the rotation member, and the second detection means detects the absolute angle from the neutral position of the steering wheel in cooperation with the movable member. Therefore, noise can be reduced by transmitting power from the rotation member to the screw shaft by meshing the gears of these members, and also the resolution of absolute angle detection can be improved by suitably setting the gear ratio of the first gear to the second gear and the pitch of the spiral groove of the screw shaft.
When the axial direction of the screw shaft is set parallel to a center axis of the rotation member in the above constitution, the steering angle detector can be reduced in size advantageously.
Any detection means is acceptable as the second detection means as long as it can detect the amount of movement of the movable member in the above constitution. Preferably, the second detection means includes a douser having a slit, a light emitting device and a light receiving device which are opposed to each other through the douser, and the movable member moves the douser, light emitting device and light receiving device relative to one another. When this light transmission type second detection means is employed, the amount of light received by the light receiving device can be changed analogly according to a change in the relative positions of the douser, light emitting device and light receiving device, thereby making it possible to improve the detection accuracy of absolute angle. It is preferred to mount the douser on the movable member side and the light emitting device and light receiving device on a fixed member placed opposite to the movable member.
Preferably, by use of a two-piece light receiving device having a first light receiver and a second light receiver as the light receiving device in the above constitution, the ratio of the amount of light received by the first light receiver to the amount of light received by the second light receiver is changed by moving the two-piece light receiving device and the slit relative to each other. The slit preferably has a linear portion which is inclined toward a moving direction of the movable member, whereby a change in the amount of light received by the two-piece light receiving device can be detected more accurately. Particularly when the whole shape of the two-piece light receiving device is rectangular and the linear portion of the slit is made substantially perpendicular to the long sides of the two-piece light receiving device, the dead area of the two-piece light receiving device is reduced advantageously.